This invention relates to a process for the biological purification of wastewater containing organic impurities, wherein the wastewater is aerated in a reactor, in the presence of activated sludge, with a gas containing molecular oxygen, e.g., air and/or pure oxygen, and thereafter, the wastewater-activated sludge mixture is separated into purified water and settled sludge. The invention also relates to an apparatus for conducting the process.
In conventional activated sludge processes, wastewater to be treated is fed into and mixed with activated sludge in a reactor designed as a continuous flow-stirred tank. Simultaneous to this feed and mixing, oxygen is introduced into the reactor to meet the oxygen demand requirements of the aerobic microorganisms in the activated sludge. As a result of the activity of these aerobic microorganisms, the organic contaminants of the wastewater, present in dissolved or colloidal form, are partially converted into bacterial substance and partially reacted, by means of oxygen, essentially into CO.sub.2 and water. Subsequently, the activated sludge is, in most cases, separated in a separator from purified wastewater by settling in a post clarification stage arranged downstream of the oxygenation tank. At least a portion of the sludge settled in the post clarification stage is recycled as recycle sludge into the oxygenation tank to maintain a desired minimum sludge concentration in said reactor.
In order to achieve adequate purification of the wastewater, it is necessary, in accordance with the prior art process, to maintain as high a sludge concentration in the oxygenation tank as technically feasible. However, due to the limited thickening, i.e., due to the settling characteristics of the biological floc, of the activated sludge, sludge concentrations of only at most 2-5 g/l can be obtained in the conventional oxygenation tanks. As a result, it is necessary that a correspondingly large amount of recycle sludge must be recycled into the oxygenation tank, resulting in high energy consumption for pumping. Furthermore, with a high sludge concentration in the oxygenation tank, the load in the post clarification stage is correspondingly high, so the post clarification tanks must be made of relatively large dimensions.
As an alternative, U.S. Pat. No. 4,419,243 discloses using reticulated carrier particles forming a skeleton for an integral mass of biomass growth to increase the concentration in a reactor. Excess biomass growing out from the skeletal structure is shed by abrasion between the carrier particles resulting in a small amount of free sludge in the wastewater. Typically this amount is on the order of only about 200 mg/l in the case of a typical municipal wastewater inflow having a BOD of about 400 mg/l. This presents problems inasmuch as due to the small amount of unfixed sludge, conventional secondary settling tanks cannot be employed because insufficient flocculation is achieved. This is further complicated by the requirement therein that in operation, the time of the carrier in the reactor must be calculated so that they are removed at the time the biomass begins to grow on the outside surface of the carriers and break off.